Some electronic laser printers use scanning laser light sources to discharge a photoconductor in the laser printer. The laser beam strikes the photoconductor and, in the well known manner, discharges the drum, leaving a charge pattern on the surface of the photoconductor in the shape of the information to be printed. Then the drum is developed with toner which is transferred to an output document. A typical laser printer is of the on/off type with no capability for gray scale output. In other words, the photoconductor either is insulating or, upon exposure to light, conducting. However, newer photoconductors have the capability of discharging only a part of the stored charge. This presents the opportunity for gray scale output because with varying levels of charge, varying levels of toner can be made to adhere to the area, which would produce a gray scale image upon transfer of the image to paper or other medium.
Thus, it is sometimes desirable to selectively discharge an electronic printer photoconductor to one of several different charge levels. To do this with a laser printhead, the exposure must be varied by changing either the intensity or the exposure time of the scanning beam. That is, the brightness of the scan beam must be capable of change, in order to discharge more or less of the photoconductor, or the scanning rate of the beam must be controllable so that the beam can be made to stay at various output locations varying amounts of time depending on the desired discharge rate for the photoreceptor. Since photoconductor velocity through the scanning region of the printer or copier is usually fixed because of synchronous speed requirements, the laser beam intensity usually must be varied. However, problems in the prior art were encountered in modulating the intensity at the video data rate and in maintaining the required intensity uniformity at each level across the format. Combining two or more beams to form one intensity variable beam also is difficult since two closely spaced beams will interfere with each other to form artifacts or patterns on the output copy.